An all day bus tour outside of Dublin

Tour companies take you to the most popular sites, so it’s really up to you and your own interests. All the tours will be “must see.”
You might want to choose a tour that’s closer to Dublin (Powerscourt/Wicklows?) rather than one that’s farther away (Cliffs of Moher, etc.), so your time can be spent touring instead of sitting on the bus.

Have fun! It’s very nice to let someone else do the driving.

Viator don't operate tours, they sell on other companies' tours with an extra mark-up. Find out the real operators and leave out the middleman.

Agree with FSS re Viator, they do not actually operate tours. If you are interested in ancient Ireland a tour of the Boyne Valley is a nice day tour out of Dublin.

My first time to Dublin I took an all day trip through Bus Eireann to Boyne Valley. The trip included stops at Hill of Tara and Newgrange as well as briefer pauses at Trim and Battle of the Boyne. I picked up tickets (the afternoon before) and caught the bus at the main Busarus station downtown.

Newgrange was a highlight of the trip.

This trip my girlfriend was coming so I wanted to take the same trip. I went to the Busarus station several times but the information window was always closed. The Bus Eireann website shows they still run tours but I've found European websites can often be completely out of date - so wanted to actually talk to someone and didn't try booking online.

Since I never was able to confirm the trip with them, I called Mary Gibbons and booked her Newgrange tour. I've seen people here mention her.

Having been to both, I'd recommend Bus Eireann as both the better tour, better pace and better value. I'd only give the nod to Mary Gibbons if you want a tour that doesn't last as long.

If you can't find anything else, Marry Gibbons tour works but I didn't think it was outstanding.

As others have said, Viator exists to book you with tours that someone else is running.

We took a tour to Glendalough and Killkenny-a worthwhile trip, but I forget the name of the tour company.

I believe the guide we used was arranged by boynevalleytours.com. A private full day tour that included several sights and great commentary. My favorite was seeing the ancient Celtic crosses at Monasterboice.

Check out Wild Rover tour company. My mom and I used them for several tours a few weeks ago. Paddywagon is another company I have used in the past. If you browse their websites, you will get ideas of what day trips are available from Dublin and the timing of the itineraries.